There is fog on the mountain a fair bit, but it isn't always thick enough to make riding not fun.
My first experience with the fog there was my first snowboarding trip ever and my first time up the tram. Imagine hearing all of these horror stories about the steep dangers at JH and then having to ride town from the top of the mountain in 10-15 ft visibility when you barely know how to ride a snowboard..... ha-ha good times.
The fog can be a mixed blessing though... sometimes it means that the conditions at the top (above the clouds, depending on what the weather is doing) are spectacularly awesome.
here are two photos for your perusal. both of these photos are from the same trail (different spots, though) atop Rendzevous mtn at JH.
Not-awesome fog at JH - http://riftwave.net/phpalbum/main.ph...jpg&var2=0_100
Awesome fog at JH - http://riftwave.net/phpalbum/main.ph...jpg&var2=0_100